The present invention relates to methods and apparatus for use by a dental practitioner to indicate when the axis of a dental bur or stone attached to a dental handpiece which the practitioner guides deviates from a required angular alignment.
Cast metal or porcelain crowns and inlays are restorations that are frequently needed for extensively damaged teeth. They are also used as retainers for fixed partial dentures (bridges). Each restoration is made in a dental laboratory on an exact model of a tooth which has been prepared for the restoration by a dentist. Once constructed, the restoration is returned to the dentist and cemented into place. Of critical importance to the success of the restoration is the convergence angle (that is the angle between opposing faces) or taper of the tooth preparation. If the walls of a tooth prepared for a crown converge towards the root of the tooth (negative taper or undercut) the restoration cannot be seated as it is "locked out". However, if too great a taper is given to the preparation, the appliance may become dislodged during function. This is because a truly adhesive dental cement is not yet available and crowns are retained by a friction developed by the cement between the prepared tooth and the internal surface of the restoration. The relationship between convergence angle and retention has been studied experimentally and is hyperbolic in nature, with half the retention lost as the taper increases from 5.degree. to 10.degree.. There is very little retention over 20.degree.. Because of this most dental textbooks recommended a taper of around 6.degree., this being said to be the least practical taper without the risk of undercuts. The desired taper is imparted either by attempting to hold a rotary cutter with the desired taper at a constant angle to the tooth, or by varying the angle of the handpiece whilst using a cylindrical cutter.
At present most dental practitioners have great difficulty in realizing this measure of accuracy with the freehand techniques used at present. Ohm and Silness ("The convergence angle in teeth prepared for artificial crowns". J. Oral Rehabil. 5, 371 (1978)) measured the convergence angle of dies sent by dental practitioners to commercial laboratories and concluded that most preparation fell into the range of 12.degree. to 37.degree. rather than the desired 6.degree.. The problem has been confirmed by other studies both in the UK and the USA. These studies concluded that the tooth preparations were commonly found to contain undercuts, overtapered surfaces and lack of a precisely defined path of insertion and that practitioners hardly ever reached the ideal (taper) described in the literature.
Not only is achieving a satisfactory taper by visual means along extremely difficult but assessment of the taper has been shown to be also very difficult. This may account for the observation that lack of retention is the second most common cause of failure of crowns and fixed partial dentures, caries being the most common.
A number of paralleling devices are known but none has been very successful. Some instruments work by temporarily fixing the dental handpiece in relationship to the teeth, movement being limited by a linked parallelogram device. In practice they have proved awkward and time consuming to align and assemble, as well as uncomfortable and intimidating to the patient.
Another group of devices is aimed at helping the dentist align the dental handpiece or at helping the assessment of the tooth preparation. One such device consists of a mirror scribed with parallel lines, another a laboratory made plastic base plate with parallel rods and another of a spirit level attached to the dental handpiece. These devices have the advantage that they do not interfere with the normal operating of the handpiece, but they are of limited value due to difficulties in feeding back the information to the dentist. For example, it is difficult to monitor the spirit level device while concentrating on the surgical procedure.